Optical amplification method and device usable with bands other than the C-band

ABSTRACT

An optical amplifier includes an optical amplification medium, an excitation source to stimulate the amplification medium to output at least one wavelength gain peak, and a gain equalizer to equalize the output of the amplification medium such that gain is produced at wavelengths other than the wavelength gain peak. The gain equalizer may attenuate gain at the peak wavelength. The gain equalizer may equalize the output of the amplification medium such that gain is produced at wavelengths less than the wavelength gain peak. The optical amplifier may include both a gain equalizer and automatic level control circuitry to respectively maintain substantially uniform gain at wavelengths within an optical signal band and maintain constant output power.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and hereby claims priority toJapanese Application No. 046467 filed on Feb. 23, 2000 in Japan, thecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] Wavelength division multiplexed (WDM) amplifiers amplify opticalsignals that are composites of multiple wavelength optical signals. WDMoptical communications systems relay multi-wavelength composite opticalsignals through multiple optical amplifiers.

[0003] The band over which losses are low in optical fiber transmissioncircuits (less than approximately 0.3 dB/km) is the band from 1450 nm to1650 nm. As shown in FIG. 1, a variety of optical fiber amplificationdevices have been developed for this transmission band.

[0004] At present, with the popularity of cellular telephones and therapid increase in internet use, the demand for telecommunicationscapacity is expanding explosively. There are global intense research anddevelopment efforts for technologies that can increase the informationtransmission capacity on a single fiber.

[0005] Optical wavelength division multiplexing (WDM) technology thatuses the broadband characteristics of optical fiber amplifiers havingsilica erbium-doped fibers (EDF) is critical. The conventionalwavelength band is known as both “the 1550 nm band” (1530 to 1560 nm) or“the C band” (conventional-wavelength band).

[0006] In addition, EDF optical amplifier equipment for a 1580 nm band(1570 to 1600 nm) “the L band” (longer-wavelength band) has beendeveloped. The competition has become intense in developing a commercialoptical fiber telecommunications system that is able to transmit anultra large capacity (perhaps 1.6 terabit/s) of information bymodulating each multiplexed wavelength at 10 Gb/s with about 80 waves ineach of the bands for a total composite of 160 waves.

[0007] Because there is a capacity of approximately eight THz when Cband and L band are combined, when 10 Gb/s transmission signal channelsare established with the 2.5 GHz spacing, the overall transmissioncapacity of 1.6 terabit can be expanded further up to 3.2 Tb/s$\left( \frac{{10\quad {Gb}\text{/}{s\quad 8}}{,{000\quad {GHz}}}}{25\quad {GHz}} \right)$

[0008] On the other hand, there is demand, for even greater carryingcapacity, and so optical fiber amplification devices that have newoptical amplification bands, in addition to the current C band and Lband, are required.

[0009] In FIG. 1, even though GS-TDFA (gain-shifted thulium-dopedfluoride-based fiber amplifiers) are being developed for amplificationin the S band region from 1490 nm to 1530 nm, GS-TDFA devices have again in the region between 1475 and 1510 nm, and thus it may bedifficult for them to succeed in the portion of S band extending from1510 to 1530 nm.

[0010] In addition, the 1610 to 1650 nm band is limited to specialtyfibers that are either thulium or terbium-doped fluoride-based fibers.

[0011] In the optical amplifier devices described above, the opticalamplification medium amplifies light through excited emission, whichoccurs from population inversion of energy levels. There is also Ramanfiber amplification, which uses the non-linear effects of fibers.Because Raman fiber amplification makes use of the non-linear effects offibers, it can produce a gain in any given wavelength band by selectingthe wavelength of the stimulating light source. However, there areproblems in that the gain per unit length is small, so the opticalamplification fibers must placed every several kilometers to everyseveral dozen kilometers within the transmission line.

SUMMARY OF THE INVENTION

[0012] An optical amplifier according to one aspect of the inventionincludes an optical amplification medium, an excitation source tostimulate the amplification medium to output at least one wavelengthgain peak, and a gain equalizer to equalize the output of theamplification medium such that gain is produced at wavelengths otherthan the wavelength gain peak. The gain equalizer may attenuate gain atthe peak wavelength. The gain equalizer may equalize the output of theamplification medium such that gain is produced at wavelengths less thanthe wavelength gain peak.

[0013] A variable attenuator and automatic level circuitry may beprovided such that the automatic level control circuitry monitors atleast one of the input of the optical amplifier and the output of theoptical amplifier and maintains the output level of the opticalamplifier at a substantially constant level.

[0014] The optical amplification medium may be formed from a pluralityof amplification medium structures which together produce at least onewavelength gain peak when stimulated by the excitation source. Theamplification medium structures may be semiconductor optical amplifiers.Also, the gain equalizer may be formed of a plurality of gain equalizersegments, which together produce gain at wavelengths other than thewavelength gain peak. The gain equalizer segments may be substantiallytransparent to the pumping wavelength of the excitation source and maybe positioned with amplification medium structures positionedtherebetween.

[0015] The excitation light source may stimulate the opticalamplification medium to achieve a population inversion rate having apositive gain throughout an optical signal wavelength band. Thewavelength gain peak may be outside of the optical signal wavelengthband. The gain equalizer may attenuate the wavelength gain peak.

[0016] The optical amplification medium has an input and an output. Afeedback hoop to the excitation source may monitor the input and outputof the amplification medium and maintain a substantially constant gainwithin the amplification medium over time. Specifically, an automaticgain control circuit may be connected to monitors at the input andoutput to control the excitation source so as to maintain a constantgain within the amplification medium over time.

[0017] The optical amplification medium may be located within aresonator. The optical amplification medium has an input and an output,and the resonator may include a pair of mirrors that reflect a selectedwavelength and optical couplers provided at the input and the output ofthe amplification medium to divert a portion of the light emitted fromthe optical amplification medium to the mirrors. The optical couplersmay be 9:1 couplers. The mirrors may be fiber grating mirrors. The gainequalizer may be substantially transparent to the selected wavelength.The selected wavelength reflected by the mirrors may be within a signalband used for optical signals, as long as no optical signal to beamplified is transmitted at the selected wavelength.

[0018] The optical amplification medium may have a cladding, a dopedcore provided interior to the cladding, and gratings provided within thehighly doped core.

[0019] Another aspect of the invention may have an amplification mediumformed of at least one erbium doped fiber, an excitation light source toproduce a population inversion ratio of about 0.7 to about 1.0 withinthe amplification medium, and a gain equalizer to obtain substantiallyidentical wavelength characteristics for a wavelength band of from about1490 nm to about 1530 nm. The excitation light source may supply pumpinglight to the amplification medium at a pumping wavelength, such that thegain equalizer is substantially transparent to the pumping wavelength.For a wavelength band of from about 1450 nm to about 1490 nm, apopulation inversion ratio of about 0.8 to about 1.0 may be used. For awavelength band of from about 1610 nm to about 1650 nm, a populationinversion ratio of about 0.3 to about 1.0 may be used.

[0020] According to an optical amplification method, a populationinversion ratio is selected to achieve positive gain throughout anoptical signal wavelength band. The amplification medium is excited tothe selected population inversion ratio to produce a wavelength gainpeak at a wavelength outside of the optical signal wavelength band. Gainis equalized to achieve substantially uniform gain over the opticalsignal wavelength band. Amplification in wavelength bands outside of theoptical signal wavelength band is attenuated. The optical signalwavelength band may be at wavelengths less than the wavelength of thewavelength gain peak for the amplification medium.

[0021] According to yet another aspect of the invention, a WDM splitterseparates first and second different optical signal wavelength bands(for example, the C-band and the L-band). An optical amplificationdevice for the first wavelength optical signal band includes a firstamplification medium, an excitation light source to produce a firstpopulation inversion ratio within the first amplification medium, and again equalizer to obtain substantially uniform gain over the firstoptical signal wavelength band. An optical amplification device for thesecond wavelength band includes a second amplification medium and anexcitation light source to produce a second population inversion ratiowithin the second amplification medium. The first and second populationinversion ratios are different. A WDM coupler recombines the first andsecond optical wavelength bands after amplification.

[0022] The first population inversion ratio may be larger than thesecond population inversion ratio, for example, assuming that the firstoptical wavelength band is the C-band and the second is the L-band. Thefirst and second optical amplification mediums may each be formed of arare earth element doped optical fiber. In this case, the length of therare earth element doped optical fiber for the first amplificationmedium may be greater than that for the second amplification medium.

[0023] The first amplification medium may have a wavelength gain peak,outside of the first optical signal wavelength band. The WDM splittermay separate first, second and third different optical signal wavelengthbands. In this case, the optical amplifier includes an opticalamplification device for the third wavelength band.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will be readily understood by reference to thefollowing description of preferred embodiments described by way ofexample only, with reference to the accompanying drawings in which likereference characters represent like elements, wherein:

[0025]FIG. 1 shows the optical transmission bands and the wavelengthranges for optical fiber amplifiers;

[0026]FIG. 2 shows the wavelength gain characteristics in terms ofrelative gain coefficients, for silica erbium-doped fiber (EDF)amplifiers;

[0027]FIG. 3 shows the gain/wavelength characteristics for the 0.9population inversion ratio shown in FIG. 2, and shows the per-unitlength gain characteristics when the S band gain is extracted andequalized;

[0028]FIG. 4 shows the wavelength characteristics of a gain equalizer inthe S band;

[0029]FIG. 5 schematically shows an optical amplifier according to afirst preferred embodiment of the present invention;

[0030]FIG. 6 schematically shows a multiple segment optical amplifieraccording to a second preferred embodiment of the invention;

[0031]FIG. 7(a) shows a highly doped optical amplification medium;

[0032]FIG. 7(b) schematically shows an optical amplifier employing agrated fiber amplification medium;

[0033]FIG. 8 shows the variation of wavelength gain characteristics withchanges in excitation current for the semiconductor optical amplifier ofFIG. 7;

[0034]FIG. 9 schematically shows an optical amplifier using a pluralityof semiconductor optical amplifier as the amplification medium,according to a third preferred embodiment of the present invention;

[0035]FIG. 10 schematically shows an optical amplifier with two fibergrating reflective mirrors to contain the optical amplification mediumwithin a Fabry-Perot oscillator, according to a fourth preferredembodiment of the present invention;

[0036]FIG. 11 shows the gain characteristics imparted to 1552 nm C bandlight by lasing at 1530 nm;

[0037]FIG. 12 is a graph showing the differences in gain imparted to1552 nm C band light for amplification with and without lasing at 1530nm; and

[0038]FIG. 13 schematically shows a broad band optical amplifieraccording to a fifth preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039]FIG. 2 shows the wavelength gain characteristics in terms ofrelative gain coefficients, or gain coefficient (dB/m), for silicaerbium-doped fiber (EDF) amplifiers. The population inversion rates aredefined by the proportion of the erbium ions that are excited. The rateis 1.0 when all of the ions are excited (i.e., when all electrons areexcited to a higher level), and if none of the ions are excited (i.e.,all are in the non-excited level), then the population inversion rate is0.0. The relative gain coefficient is labeled on the vertical axis asthe gain per unit length.

[0040]FIG. 2 corresponds with FIG. 3 of Y. Sun, J. L. Zyskind and A. K.Srivastava, “Average Inversion Level, Modeling, and Physics ofErbium-Doped Fiber Amplifiers,” IEEE Journal of Selected Topics inQuantum Electronics, Vol. 3, No. 4, pp. 991-1007, August 1997. Therelative gain coefficients shown in FIG. 2 are applicable to any of aplurality of amplification mediums. The FIG. 2 relative gaincoefficients can be converted to the Sun et al. gain coefficients (dB/m)if an erbium-doped fiber amplification medium is specified. Also, thepopulation inversion rate of FIG. 2 corresponds with the inversion levelin the FIG. 3 of the Sun et al. article. The definition of thepopulation inversion rate or inversion level is the same as that givenby Eq. (22) of Sun et al. If the inversion rate is averaged along thelength of the EDF, an average inversion rate is obtained, which averageinversion rate is defined by Eq. (23) of the Sun et al. article. Infiber amplifiers, the average inversion rate given by Eq. (23) isimportant. The importance is at least in part due to the fact that theamplifier gain is defined in Eq. (20) of Sun et al. in terms of theaveraged rate (ΔN(t)) and the length (L). Therefore, in the discussionof fiber amplifiers, the “population inversion rate” refers to theaverage population inversion rate.

[0041] At present, multi-wavelength optical fiber amplifiers usingsilica-based erbium-doped fibers (EDF) have population inversion ratesup to about 0.7 within the 1550 nm band (1530 to 1570 nm). As can beseen from FIG. 2, the gain is not equal at different wavelengths.Therefore, a gain equalizer is used to produce an even gain that isindependent of the wavelength.

[0042] As with the C band, multi-wavelength optical amplifiers thatamplify the long-wavelength L band (1560 to 1610 nm) are nearing thelevel for commercialization. In the C band optical amplifier, the gainshape corresponding to a population inversion rate of 0.7, assuming anaveraged inversion rate in this discussion, is employed. In the L bandoptical amplifier, the population inversion rates is intentionallydropped to about 0.4 in silica-based erbium-doped fibers (EDP) toproduce maximized gain in the L band. This L band optical amplifier,which uses a silica-based erbium-doped fiber (EDF), requires a longerEDF length, because the gain coefficient (dB/m) is smaller, as shown inFIG. 2.

[0043] When one looks closely at the wavelength gain characteristicsshown in FIG. 2, when the population inversion ratio is set to, forexample 0.9, in the C band, high gains are achieved in bands where theyhave not been achieved before, such as in the 1450 to 1530 nm band andthe 1610 to 1650 nm band (high compared with the gains achieved in the Lband using an inversion rate of 0.4). However, because the per-unitlength gain for the C band (1530 to 1570 nm) is large when the C bandpopulation inversion ratio is 0.9, amplification is dominated by the Cband. Because of this, the inventors proposed a gain equalizer (GEQ) tosuppress the C band.

[0044]FIG. 3 shows the gain/wavelength characteristics for the 0.9population inversion ratio shown in FIG. 2, and shows the per-unitlength gain characteristics when the S band gain is extracted andequalized. When the population inversion ratio is at 0.9, there is again peak in the vicinity of 1530 nm. When the area within the gaincurve marked by the diagonal lines is eliminated, then there will begain equalization such that, in the S band, there will be flat gaincharacteristics as shown by the white cutout rectangular area. The otherwavelength bands are eliminated. Using this method, not only is thelarge per-unit-length gain in the C band eliminated by the equalizer,but also the per-unit-length gain in the S band is reduced because ofthe equalization. The gain in the S band is flattened for ease in wavelength division multiplexing of the transmitted signal light.

[0045] Though the per-unit-length gain for the L band optical amplifiersthat amplify light using the 0.4 population inversion ratio of FIG. 2 issmall, practical L band amplifiers are realized. In other words, whenone considers new band amplifiers, we can see that amplification of anyband, which has a positive gain coefficient at a selected inversion ratecan be realized using the silica-based EDF. Both the gain equalizer andthe length of the optical amplification medium are selected based on thegain characteristics at the desired population inversion ratio in orderto produce a practical gain in the S band, the S+ band, and the L+ bandusing silica-based erbium-doped fibers (EDF).

[0046]FIG. 4 shows the wavelength characteristics of a gain equalizerGEQ, an “optical filter,” in the S band. The structure is such thatthere is a transmittance region in the band between 950 nm and 1000 nmso that 980 nm excitation light can pass through. In the 1490 nm to 1530nm-band, the transmittance falls as the wavelengths grow longer in orderto produce the gain characteristics of the white cut out rectangle ofFIG. 3. The transmittance in this band varies inversely with theunequalized amplification gain.

[0047] Looking at FIG. 4 in terms of FIG. 2, FIG. 4 shows a structurethat suppresses the gain peak centered at 1531 nm, which is thecharacteristic peak. However, if the EDF population inversion is varied,there could be a gain peak in the S band. The gain peak can vary betweenabout 1528 nm and 1535 nm. In addition to the population inversionratio, the gain peak varies depending on the dopant material (materialssuch as Al or Ge) and depending on the effective cross-sectional area ofthe fiber.

[0048] Consequently, because the gain peak central wavelength will vary,the characteristics of the gain equalizer must be matched to the centralwavelength of the gain for the selected amplification medium and for thepopulation inversion ratio.

[0049]FIG. 5 schematically shows an optical amplifier according to afirst preferred embodiment of the present invention. In FIG. 5,reference numeral 1 represents a silica-based erbium-doped fiber (EDF),reference numeral 2 represents a gain equalizer (GEQ), referencenumerals 31 and 32 represent optical isolators, reference numeral 4represents the excitation light source, reference numeral 5 represents amulti-wavelength coupler, reference numeral 8 represents the inputterminal, reference numeral 9 represents the output terminal, referencenumerals 71 and 72 represent optical splitter couplers, referencenumeral 81 represents an input monitor photodiode PD, reference numeral82 represents an output monitor photodiode PD, and reference numeral 50represents an automatic gain control circuit (AGC).

[0050] Multi-wavelength light that is input through input terminal 8 topass through the optical isolator 31 and the multi-wavelength coupler 5.This light is input into the silica-based erbium-doped fiber (EDF) 1,which is the amplification medium that produces excited emission. Thesilica-based erbium-doped fiber or fibers (EDF) used here has/have a 7mm mode field diameter, an Er density of 500 ppm and a fiber length of150 m. This fiber configuration is only one example, and other typicalEDFs can be used. Typical EDF mode field diameters of fibers currentlyon the market range from 5 mm to 8 mm, and typical Er densities rangefrom 100 ppm to 1500 ppm. When it comes to fiber length, the length isadjusted for the amplifier depending on the amplification gain and onthe Er density. The length can vary within a broad range, from about 1 mto about 10 km. Furthermore, the fiber length is subject to adjustmentdepending on the desired gain and the per-unit-length gain for theamplified wavelength band, which depends on the population inversionratio.

[0051] In the silica-based erbium-doped fiber (EDF) 1, themulti-wavelength light that is injected through input terminal 8 isoptically amplified using 0.98 μm excitation light from the excitationlight source 4. The excitation light is injected through themulti-wavelength coupler 5. After amplification, the injected light issent to the gain equalizer 2. The excitation light power is controlledby AGC (50) so that the average population inversion ratio in thesilica-based erbium-doped fiber 1 is 0.9 and the wavelengthcharacteristics in FIG. 3 are obtained.

[0052] The gain equalizer 2 may have the gain equalizationcharacteristics shown in FIG. 4, in which gain equalization producesgain within the white rectangular cutout in FIG. 3. In FIG. 3,transparency of the gain equalizer 2 to excitation (or pump) band lightis not required, because the gain equalizer 2 is located at the outputport of the silicon-based erbium-doped fiber (EDF) 1. That is, the gainequalizer 2 need not be transparent to 980 nm excitation light in thiscase. The gain equalizer 2 can be made from combinations of multipleFabry-Perot etalon filters, multilayer dielectric filters, and/or fibergrating filters.

[0053] The amplified signal travels to the output (9) through the gainequalizer 2 via the isolator 32 and the optical splitter coupler 72. Theoptical splitter coupler 71 on the input terminal side splits a portionof the input light to supply the input monitor photodiode PD 81. Theoptical splitter coupler 72 on the output terminal side splits a portionof the amplified output light and supplies it to the output monitor PD82.

[0054] The automatic gain control circuit (AGC) 50 controls the opticaloutput power of the 0.98 μm semiconductor laser, which that serves asthe excitation light source 4. Control is based on the light detected atthe input monitor PD 81 and the output monitor PD 82 so that the gain ofthe erbium-doped fiber 1, or the averaged population inversion ratio,remains constant. By maintaining the EDF gain at a constant value thepopulation inversion ratio is also maintained at a constant value,regardless of the input power.

[0055] When the output level should be controlled to a constant valueand more control, namely an automatic level control (ALC) is desired, avariable attenuator can be provided in addition to having the automaticgain control circuit (AGC) 50. The variable attenuator can be positionedat either the input terminal 8 or the output terminal 9, making itpossible to control the output of the optical amplifier to a constantvalue by controlling either the level of the optical signal that isinput to the optical amplifier or by controlling the level of theoptical signal that is output from the optical amplifier. This controlis appropriate when controlling the gain of the optical amplifier to aconstant value is not sufficient because of optical signal powerfluctuations due to variations such as span-loss variation. Therefore,automatic gain control is used to keep theaveraged-population-inversion-level constant, and automatic levelcontrol is used to keep the total-output-power constant.

[0056] The reason why the gain control of the automatic gain controlcircuit (AGC) 50 is helpful is because the wavelength characteristics ofthe gain or the averaged population inversion ratio can be kept at acertain level, as is shown in FIG. 2.

[0057] The reason why the 0.98 μm excitation light source is used inFIG. 5 is because it is able to raise the population inversion ratio upto 1, while 1.48 μm pump light is theoretically limited to a populationinversion rate of approximately 0.7. As long as there is a sufficientaveraged inversion level, with good amplification efficiency, a 1.48 μmband (ranging from 1.45 μm to 1.49 μm) pump light can be used instead.

[0058]FIG. 5 has been explained with regard to forward-pumping where thepump light is supplied from the input terminal side of the EDF. However,the multi-wavelength coupler 5 can be positioned between the gainequalizer 2 and the optical isolator 32 to excite in the backwardsdirections, stimulating the EDF from output terminal side.Alternatively, bi-directional excitation, where excitation light excitesthe EDF from both the input terminal side and the output terminal side,can be used. In the case of bi-directional excitation, a combination ofthe 0.98 μm and the 1.48 μm pump light bands can be used for the pumpingsources.

[0059] When back-ward excitation is used, the wavelength characteristicsof the gain equalizer must be such that it is possible for the backwarddirection excitation light to pass through GEQ 2. FIG. 4 shows anexample of GEQ characteristics transparent to 0.98 μm pump light.Specifically, the 1.48 band excitation light or the 0.98 band excitationlight, which ever is used for backward pumping, must pass through thegain equalizer 2.

[0060] The excitation light source need not necessarily be just a singlesemiconductor laser, but a composite of wavelengths or polarizations oflight from multiple semiconductor lasers can be used.

[0061] Although a population inversion ratio of 0.9 was used as anexample for S band optical amplifier shown in FIG. 5, the populationinversion ratio can be otherwise selected to achieve gain (for adesired) band region, provided that the selected population inversionratio provides a positive gain coefficient in the desired band. See FIG.2. As with before, gain equalization can be performed to reduce the gainin bands other than the band region used. The S band optical amplifiercan use population inversion ratios ranging from 0.7 to 1 with theoptical amplification medium used in FIG. 2.

[0062] When, structuring an S+ band optical amplifier for the bandsbetween 1450 nm and 1490 nm, a population inversion ratio between 0.8and 1 can be used from FIG. 2. When structuring an L+ band opticalamplifier for the wavelengths between 1610 nm and 1650 nm, a populationinversion ratio between 0.3 and 1 can be used.

[0063] Because the gains obtained for each band will vary with thelength of the EDF (which is the optical amplification medium), thelength of the EDF must be selected to match the targeted gain.

[0064] The S band optical amplifier described above is very differentfrom the L band optical amplifiers already developed. When the gainwavelength characteristics of the L band optical amplifiers that usepopulation inversion rates of about 0.4 (FIG. 2) are examined, it isapparent that the peak gain is within the L band (even though the valueitself is small). This is because the excitation light powerautomatically is converted into an L band amplifying light if thepopulation inversion ratio is held at 0.4 by the gain control circuit(AGC). Therefore, the L band amplifier has a high amplificationefficiency in the band of light being amplified.

[0065] On the other hand, when S band amplification is performed using apopulation inversion ratio of 0.9, it is necessary to suppress with again equalizer, the C band light, for example, where the gain is largerthan for the S band.

[0066] The GEQ 2 of FIG. 5, is provided only on the output side of thefiber 1. In this case the conversion efficiency of the optical amplifiermay be less than desired because the majority of the excitation lightpower is converted into spontaneous emission (ASE) outside of the Sband, mainly around the 1.53 μm gain peak in FIG. 3. In FIG. 5 the gainequalizer eliminates this enormous and unnecessary ASE. The conversionefficiency of the amplifier in FIG. 5 may be only several percent orlower. In general, the conversion efficiency of optical amplifiers inthe C band is about 60%, and in the L band, conversion efficiencies ofabout 40% have been achieved.

[0067]FIG. 6 shows an example of an embodiment for improving theconversion efficiency relative to the structure in FIG. 5. FIG. 6schematically shows a multiple segment optical amplifier according to asecond preferred embodiment of the invention. In FIG. 6 theamplification medium is divided into multiple segments. For example, ifthe sum total length of these segments equals the length of the EDF inFIG. 5, then the FIG. 6 device achieves the same gain as in theamplifier of FIG. 5. In FIG. 6, gain equalization is distributed alongthe entire amplification medium to improve the conversion efficiency byeliminating ASE before it becomes very large.

[0068] In FIG. 6, reference numerals 11, 12, and 13 representsilica-based erbium-doped fibers (EDF), reference numerals 21, 22, and23 represent gain equalizers (GEQ), reference numerals 31 and 32represent optical isolators, reference numeral 4 represents anexcitation light source, reference numeral 5 represents amulti-wavelength coupler, reference numeral 8 represents the inputconnector, reference numeral 9 represents the output connector,reference numerals 71 and 72 represent optical splitter couplers,reference numeral 81 represents an input monitor PD, reference numeral82 represents an output monitor PD, and reference numeral 50 representsan automatic gain control circuit (AGC).

[0069] The wavelength multiplexed light is injected from the input port8 to pass through the optical splitter coupler 71, the optical isolator31, and the multi-wavelength coupler 5. From there, the input light isinjected into the first silica-based erbium-doped fiber (EDF) 11, whichserves as the amplification medium. If an EDF length of 50 m isappropriate to obtain the desired gain using a population inversionratio of 0.9, this EDF length can be segmented into one meter lengthsegments by using fifty silica-based erbium-doped fibers, the first EDFfiber 11 through fiftieth EDF fiber 13 (EDF50). Each EDF segment isconnected, respectively, to one of the gain equalizers from the firstGEQ′1, 21 through fiftieth GEQ′50, 23. The prime symbol (′) after “GEQ”is to differentiate the gain equalizers of the FIG. 5 and FIG. 6embodiments. A long-period fiber grating gain equalizer is a goodcandidate for the gain equalizers GEQ′ 1, GEQ′ 2, . . . , and GEQ′ 50.

[0070] The GEQ′ wavelength transmittance characteristics can be reducedto 1/50 of the required transmittance (in terms of dB units) for thesignal wavelength band (e.g., 1490 to 1530 nm). This is because fiftyGEQ′ units are used. Therefore, the small amount of ASE produced in eachsegment (perhaps 1 meter long) is eliminated at the output of each EDFsegment by a gain equalizer GEQ′. In this manner, the unnecessaryconversion of pump light power to ASE is dramatically reduced.

[0071] In the silica-based erbium-doped fibers EDF, the multi-wavelengthlight that is injected through the input terminal 8 is amplified by the0.98 μm excitation light from the excitation light source 4. Theexcitation light is injected through the multi-wavelength coupler 5.From EDFs, the multi wavelength light is injected into the gainequalizers. Within EDFs, the excitation light in the silica-basederbium-doped fiber 1 performs excitation to obtain the wavelengthcharacteristics at a population inversion ratio of 0.9. Exemplarywavelength characteristics are shown in FIG. 3.

[0072] The gain equalizers 21 through 23, as a whole, have the gainequalization characteristics shown in FIG. 4, to impart thecharacteristics shown by the white rectangular cutout in FIG. 3. Thegain equalizers 21 through 23 are substantially transparent with respectto the pumping light. The gain equalizers 21 through 23 should have highreturn-loss characteristics, namely very low reflectivity (˜−60 dB) toavoid resonance among the gain equalizers 21, 22 and 23. After gainequalization, the amplified light is output through the optical isolator32, with the amplified light exiting via the output terminal 9.

[0073] Each of the gain equalizers 21-23 may have its own uniqueequalization characteristics or the same equalization characteristics.Regardless of the individual equalization characteristics, thecharacteristics obtained as the final result should be the same as thecharacteristics shown in FIG. 4. The gain equalizers 21 through 23 canbe created from a combination of Fabry-Perot etalon filters anddielectric multilayer filters, or by fiber grating filters.

[0074] The input-side optical splitter coupler 71 splits off a portionof the incident light and injects it into the input monitor photodiodePD 81, while the output-side optical splitter coupler 72 splits off aportion of the light that was amplified by the last EDF 13 and injectsit into the output monitor photodiode PD 82.

[0075] The automatic gain control circuit (AGC) 50 controls the opticalpower that is output from the 0.98 μm semiconductor laser that serves asthe excitation light source 4. The output power is controlled based onthe light that detected by the input monitor PD 81 and the outputmonitor PD 82 to maintain the gain of the optical amplifier (or, morestrictly speaking, the total gain of all EDFs) at a constant value. Gaincontrol is important because variations in the gain cause variations inthe wavelength characteristics due to the variations of the averagedpopulation inversion rates.

[0076] In addition, when it is desirable that the automatic levelcontrol (ALC) be performed so that the output level is constant, whilemaintaining constant the wavelength characteristics of the gain by AGC(50), a variable attenuator can be added at the input terminal 8 or theoutput terminal 9. With a variable attenuator, the output of the opticalamplifier can be maintained at a constant level through controllingeither the level of either the optical signal injected into the opticalamplifier or the optical signal output from the optical amplifier. Thiscan occur even if the gain of the optical amplifier is controlled to aconstant gain.

[0077] In FIG. 6, a 0.98 μm excitation light source is used. However, a1.48 μm excitation light source may be used instead.

[0078] Furthermore, FIG. 6 has been explained with regard to forwarddirection excitation, to excite the EDF from the input side. However,the multi-wavelength coupler 5 can be equipped between the opticalisolator 32 and the gain equalizer 23 to allow backwards excitation fromthe output terminal side of the EDF. Alternatively, bi-directionalexcitation can be used, by providing excitation light to the EDF fromboth the input terminal side and the output terminal side of the EDF.Furthermore, when bi-directional excitation is used, a 0.98 μmexcitation light source and/or a 1.48 μm excitation light source can beused. In this case, the excitation light with either wavelength can beused as the forward excitation. The excitation light source need notnecessarily be just a single semiconductor laser, but a composite ofwavelengths or polarizations of light from multiple semiconductor laserscan be used.

[0079] In addition, when it is necessary for the population inversionratio to be high, such as close to 1, then high power excitation lightsource is required and multi-wavelength couplers can be equipped betweeneach EDF segment. In this manner, excitation light (forward excitation,backwards excitation, or bi-directional excitation) can then be suppliedseparately to each EDF segment.

[0080] Although a population inversion ratio of 0.9 may be used toproduce the S band optical amplifier of FIG. 6, other populationsinversion ratios can be selected to produce gain in the desired band. Aswith the S band and the 0.9 inversion ratio, gain equalization can beperformed to reduce the gain in bands other than the desired band.

[0081] An S band optical amplifier can be structured using populationinversion ratios ranging from 0.7 to 1 with the optical amplificationmedium used in FIG. 6. An S+ band optical amplifier for the bandsbetween 1450 nm and 1490 nm, can be achieved with a population inversionratio between 0.8 and 1.0. An L+ band optical amplifier, for the bandsbetween 1610 nm and 1650 nm, can be achieved with a population inversionratio between 0.3 and 1.0.

[0082] In the section below, the device shown in FIG. 5 will be comparedwith the device shown in FIG. 6 to explain the improvement in theconversion rate of the excitation light power into S band signal light.

[0083] In the optical amplifier shown in FIG. 5 the ASE gain curve shownin FIG. 3 is large. Because the portion that is shown with the diagonallines (most of the gain) is eliminated by the GEQ 2, the excitationlight converted into unused ASE represents somewhat of a waste.

[0084] On the other hand, even though the pump light source 4 shown inFIG. 6 produces the same ASE gain curve of FIG. 3, each gain equalizerfrom 21 to 23 eliminates ASE except at 1490 to 1530 nm, namely exceptwithin the S-band. Before subsequent amplification stages, unneededlight is removed. This leads to improved efficiency.

[0085] If we assume that the total ASE power in FIG. 3 (i.e., the totalarea under the curve) is 100 mW, and we eliminate the part shown by thediagonal lines (which we assume to be 90%), then about 90 mW is wasted.The wasted 90 mW has been converted from the excitation light.Therefore, at the very least, 90 mW of power of the excitation light iswasted.

[0086] Because the white rectangular cutout area is in the signal band,it cannot be eliminated. In FIG. 6, there are 50 EDF segments. Perhaps1/50 of the ASE gain occurs in each EDF segment—1/50 of the total ASEgain power (area under the FIG. 3 curve). For a total ASE power of 100mW, this translates to 2 mW. If 90% is wasted, this translates to awasters of at most 1.8 mW.

[0087] The critical point here is that the 1.8 mW ASE that is generatedin the first stage erbium-doped fiber 11 (EDF1) causes unwanted excitedemission in the next stage if it is not eliminated using the GEQ 21.

[0088] Placing a single GEQ on the output side, as shown in FIG. 5,allows the waste ASE to grow along the EDF 1. The well-grown ASE at theend of EDF 1 is eliminated by the GEQ 2. On the other hand, in FIG. 6,the ASE is eliminated after it has only grown slightly. Therefore it ispossible to use for signal amplification, the excitation energy thatwould otherwise be used to amplify the unwanted ASE. This improves theconversion efficiency. Accordingly, segmenting the EDF and insertinglow-loss GEQs between the segments increases the conversion efficiency.

[0089] The key to improving the conversion efficiency is the selectionof the position of the GEQs in the lengthwise direction of the EDF. Ifthe FIG. 5 device were altered to divide the silica-based erbium-dopedfiber 1 into two segments with a gain equalizer placed between the twosegments, the characteristics shown by the rectangular cutout in FIG. 3can still be produced as the final output. With this two segmentstructure, it is possible to improve excitation light conversion in theS band over that where a single gain equalizer is placed on the outputside.

[0090]FIG. 7(a) shows a highly doped optical amplification medium, forexample, a highly-doped EDF. Although FIGS. 5 and 6 use existingsilica-based EDFs, other amplifier mediums, such as a shortened highlydoped fiber or Er-doped optical wave guide shown in FIG. 7(a), can beused. FIG. 7(a) shows a semiconductor amplification device. In thisdevice, the density of the added erbium per-unit-length is increased.Reference numeral 14 represents the core, reference numeral 15represents the cladding, and reference numeral 16 represents thegrating.

[0091] In FIGS. 5 and 6 the EDF has a mode field diameter of 7 mm anderbium is doped at a concentration of 500 ppm. A population inversionratio of 0.9 may be used, and the total length of the EDF may be about150 m. These properties obtain the target gain of about 20 dB.Consequently, if the fiber or waveguide substrate base material useshigh density Er doping of 15×10⁵ ppm (3,000 times as much doping), thena total length of 5 cm (1/3,000th the length) would be adequate toachieve the same gain. If the length traversed by the light is 5 cm,then the grating 16 can function as a gain equalizers 21, 22 and 23.Because the FIG. 7(a) device may serve as both the amplification mediumand the gain equalizers 21-23, it represents an almost infinitesegmentation compared with 50 unit segmentation in FIG. 6. The gratings16 of FIG. 7(a) are formed in the optical wave guide core 14. Bystructuring the amplification medium as shown in FIG. 7(a), it ispossible to achieve optical amplifier with excellent properties, similarto that when the GEQs are distributed such as shown in FIG. 6.

[0092] Gratings 16 can be formed in the waveguide core of asemiconductor optical amplifier or gratings can be formed in a singlemode fiber amplification medium. Gratings can also be formed in anon-highly doped silica-based erbium doped fiber. In this case, it isimportant to use technology such as the long-period grating technologyso that the eliminated light by the GEQ is not returned to the core, ormore precisely not coupled to the fundamental-mode, to cause resonance.Resonance occurs within the Er-doped amplification medium when theremoved light by the GEQ is returned into the core. This leads tounstable operation due to resonance or unwanted laser emission. It isnecessary to create and install the GEQs to avoid this situation.

[0093]FIG. 7(b) is a schematic view of an optical amplifier employing asingle mode fiber amplification medium 17 having gratings 171 formedtherein. The gratings 171 are formed in the fiber core using long-periodgrating technology. The amplification medium 17 therefore functions asboth a amplifier and as a gain equalizer. Therefore, an effect similarto having multiple gain equalizers GEQ 21, 22 and 23, is achieved. Infact, because there are numerous gratings 171 formed in the fiber 17, itis analogous to having an almost infinite number of fiber segments andan infinite number of gain equalizers in the FIG. 6 device. In the FIG.8 device, the fiber 17 with gratings 171 could be replaced with thehighly doped amplifier shown in FIG. 7(a) and having gratings 16. Theschematic appearance of the device would be similar.

[0094]FIG. 8 shows the variation of wavelength gain characteristics withchanges in excitation current for the semiconductor optical amplifier ofFIG. 7(a). So we use the FIG. 7(a) device as an EDF with gratings in onecase, and as a semiconductor amplifier with gratings in the other case.When a semiconductor optical amplifier is instead of a fiber and a pumplight source, a bias current is used as the excitation source instead oflight.

[0095] As can be seen in FIG. 8, the semiconductor optical amplifier hasdifferent amplification peaks at different population inversion ratios.By changing the population inversion ratio through varying theexcitation current, the wavelength position of the amplification peakand the gain curve are changed.

[0096] As with the use of pumping light in EDFs, the excitation currentis selected to produce a population inversion ratio in which sufficientgain is achieved and maintained constant in the band of interest. Byusing multiple gain equalizers to equalize the gain produced in unwantedband ranges, it is possible to achieve an excellent pumping lightconversion ratio in a band outside of the peak amplification gainwavelength.

[0097]FIG. 9 schematically shows an optical amplifier using a pluralityof semiconductor optical amplifiers as the amplification medium. Most ofthe components shown in FIG. 9 are the same as those in FIG. 6. However,multiple semiconductor optical amplifiers (“SOA”) 33, 34, and 35 such asthat shown in FIG. 7 are used instead of multiple erbium doped fibers.Wavelength division multiplexed signal light is injected from inputterminal 8 to pass through the optical splitter coupler 71 and theoptical isolator 31. The signal is then injected into the semiconductoroptical amplifiers (SOA) 33 through 35, which serves as theamplification media.

[0098] In the device shown in FIG. 9, a plurality of semiconductoroptical amplifiers (SOA) are connected together in a staged relationshipto obtain the specific gain in the target wavelength band. Gainequalizer 21 (GEQ′ 1) through gain equalizer 23 (GEQ′ 50) arerespectively connected between the semiconductor amplifier segments 33through 35. Because multiple gain equalizers are used, the overalltransmittance wavelength characteristics (that are achieved at theoutput) are derived from the sum of the gain equalizations achieved inthe gain equalizers. In each gain equalizer, the amount of gainequalization may equal the total amount of gain equalization divided bythe number units (see FIG. 5). Alternatively, each of the gainequalizers may have its own unique equalization characteristics.Regardless of whether they have unique or the same equalizationcharacteristics, the characteristics obtained as the final result shouldbe such that a flat gain region is obtained in the target band.

[0099] The gain equalizers 21 through 23 can be created by a combinationof Fabry-Perot etalon filters and dielectric multilayer filters, orfiber grating filters.

[0100] The input terminal-side optical splitter coupler 71 splits off aportion of the incident light and injects it into the input monitorphotodiode PD 81 and the output terminal-side output splitter coupler 72splits off a portion of the light that has been amplified by thesemiconductor amplifiers SOA 33-35 and injects it into the outputmonitor PD.

[0101] The automatic gain control circuit (AGC) 50 controls the biaslevel of the excitation current for semiconductor optical amplifiers SOA33 to 35 based on the light that is detected by the input monitor PD andthe output monitor PD to maintain constant the overall gain of theoptical amplifier.

[0102] Variations in wavelength characteristics cause variations ingain. If a variable gain equalizer (GEQ) is used, it can adapt tochanges in the SOA gain due to changes in the wavelengthcharacteristics.

[0103] In addition, when it is desirable to maintain the output levelconstant, while maintaining the wavelength characteristics of the gainby AGC (50), a variable attenuator can be added at the input terminal 8or the output terminal 9. The variable attenuator can be controlledthrough automatic level control (ALC) circuitry. With a variableattenuator, the output of the optical amplifier can be maintained at aconstant level through controlling either the level of either theoptical signal injected into the optical amplifier or the optical signaloutput from the optical amplifier. This may be appropriate even if thegain of the optical amplifier is controlled to a constant gain.

[0104]FIG. 10 schematically shows an optical amplifier with two fibergrating reflective mirrors (FG-mirror) 42, 43 to contain the opticalamplification medium within a Fabry-Perot oscillator. Silica-basederbium-doped fibers 11, 72, 23 are used as the optical amplificationmedium. With the resonator structure formed by the FG mirrors 42, 43, itis not necessary to use an automatic gain control circuit (AGC)controlled via I/O monitors.

[0105] In FIG. 10, 9:1 couplers (CPL) 73 and 74 are incorporated intothe device of FIG. 6. The fiber grating mirrors 42 and 43 are connectedto the device through 10% port of the 9:1 couplers (CLP) 73 and 74. Sothe effective reflectivity is 1/100 of the reflectivity of FG-mirror.That is, 10% of the light from EDF1 is sent to the fiber grating mirror42 for reflection. Then, 10% of the reflected light is reintroduced backto the EDF1.

[0106] The first 9:1 coupler (CPL) 73 is inserted between themulti-wavelength coupler 5 and the first silica-based erbium-doped fiber11 (EDF 1). The first fiber grating mirror (FG-mirror) 42 is connectedto the branched end of the first 9:1 coupler (CPL) 73.

[0107] The second 9:1 coupler (CPL) 74 is equipped between the gainequalizer 23 (GEQ′ 50) and the optical isolator 32. At the branched endof the second 9:1 coupler (CPL) 74, the second fiber grating mirror(FG-mirror) 43 is provided.

[0108] Below will be explained an example of how the above deviceoperates as an S band multi-wavelength optical amplifier.

[0109] First of all, preparations are done to ensure that there is nosignal light at the lasing wavelength 1530 nm. The signal light isamplified by the silica-based erbium-doped fiber 11 and receives gainequalization from the multiple gain equalizers 21-23 to achieve thedesired S band gain wavelength characteristics such as shown by thewhite rectangular area in FIG. 3. The signal light is then output.

[0110] 90% of the light amplified by the silica-based erbium-dopedfibers 11-13 is output to the optical isolator 32 by the second 9:1coupler 74. The remaining 10% of the amplified light is output to thefiber grating mirror (FG-mirror) 43.

[0111] The fiber grating mirror (FG-mirror) 43 reflects the 1530 nmlight, in a wavelength band of 1530 nm ± a few tenths of an nm. Thefiber grating mirror 43 returns the reflected light to the silica-basederbium-doped fibers 11-13 through the second 9:1 coupler 74.

[0112] The silica-based erbium-doped fibers 11-13 amplify the returnedlight. Ten percent of the light traveling on the return path is split bythe first 9:1 coupler 73 and sent to the first fiber grating mirror(FG-mirror) 42. The fiber grating mirror (FG-mirror) 42 reflects the1530 nm light (in a wavelength band of 1530 nm ± several tenths of annm), and then returns the reflected light to the silica-basederbium-doped fibers 11-13 via the first 9:1 coupler 73.

[0113] The device shown in FIG. 10 forms a 1530 nm Fabry-Perot resonatorfrom the two fiber grating mirrors 42, 43, the two 9:1 couplers (CPL)73, 74 and the EDFs 21-23 (which together serve as the amplificationmedium).

[0114] The light stimulates the EDFs to form population inversion, whichfulfills the lasing conditions at the 1530 μnm wavelength, producing anamplified 1530 nm laser output.

[0115] When this lasing occurs, the averaged population inversion ratiois fixed at a single value (and the gain is also fixed), and thus, evenif the input is changed, the gain and the wavelength characteristics ofthe gain remain constant.

[0116] When the input signal is strong, then a lot of the excitationlight power is expended in amplifying the signal light, and the laseroperation at 1530 nm stops. When the laser operation stops, the gainstops being uniform. This operation will described in more detail withregard to FIG. 12.

[0117] Even though the fiber grating creates a Fabry-Perot resonancelasing at 1530 nm, the fiber grating is not restricted to thiswavelength. Resonance can be created with lasing at any wavelength solong as the wavelength (1) causes population inversion in the opticalamplification medium, (2) can pass through the gain equalizers, (3) iswithin the S band, and (4) is not a wavelength used for signal light tobe amplified within the resonator. In addition, the resonator structureis not limited to a Fabry-Perot resonator. A ring-shaped resonator canalso be used.

[0118] Although the device of FIG. 10 has been described with regard tothe S band, other bands can be used, if the other bands (1) are within awavelength range that causes a population inversion in the opticalamplification medium, (2) can pass through the gain equalizers, (3) havea lasing wavelength therein, and (4) the lasing wavelength is not usedfor the signal to be amplified.

[0119] If the FIG. 10 gain equalizers are structured to have thewavelength characteristics shown in FIG. 4, light above about 1530 nm isdiscarded. If the gain equalizer would pass some wavelengths above about1530 nm (above the signal band), lasing could occur outside of thesignal band so long as the fiber grating mirrors 42, 43 createdresonance at the lasing wavelength.

[0120] The reasons why the gain of the signal light due to lasing isconstant are explained with regard to FIGS. 11 and 12. FIG. 11 shows thegain characteristics imparted to C band light at 1552 nm by lasing at1530 nm.

[0121] In FIG. 10, both ends of the optical amplification medium areequipped with optical couplers, where, at the end of each branch, afiber grating mirror reflects light at 1530 nm. This forms a resonatorand makes it possible to confirm lasing at 1530 nm. Note the peak inFIG. 11 at 1530 nm.

[0122]FIG. 12 is a graph showing the differences in gain imparted to Cband light at 1552 nm for amplification with and without lasing at1530nm. The gains shown in FIG. 12 were obtained from the levels ofinput and output signal light. As with FIG. 11, the signal light issupplied to the optical amplification medium at 1552 nm. Thecharacteristics marked with the circles in the diagram are thosecharacteristics where there is no resonator and no lasing. As can beseen, the gain changes with the power of the input light.

[0123] The characteristics marked with the squares are thosecharacteristics where an oscillator is constructed within the opticalamplification media, which oscillator resonates at 1530 nm, causinglasing to occur. As can be seen, with lasing, the gain is substantiallyconstant over a broad input power range from −35 dBm to −10 dBm. Thatis, fluctuations in input power do not effect the gain. FIG. 12 showsthe circumstance where the laser operation is discontinued when theinput power level is about −7 dBm or higher.

[0124]FIG. 13 schematically shows a broad band optical amplifieraccording to a fifth preferred embodiment of the present invention.Multi-wavelength optical signals come into the amplifier via atransmission path 57 made from an optical fiber. The optical fiber maybe a single-mode fiber SMF (with a zero-dispersion wavelength at 1.3μm), a dispersion-compensation fiber (a fiber that has a negativedispersion value relative to the SMF), a dispersion-shifted fiber DSF (afiber with the zero-dispersion wavelength within signal wavelengthband), or a non-zero dispersion shifted fiber NZ-DSF (a fiber with thezero-dispersion wavelength provided adjacent to the transmission signalwavelength band).

[0125] The transmission path 57 is excited by the excitation lightsource 56 through a multi-wavelength coupler 66 to perform Ramanamplification, which improves the noise figure (NF) when performingdistributed amplification on partitioned wavelength bands. In othercases, the Raman amplifier (66 and 56) is not required.

[0126] The multi-wavelength optical signals that are amplified by theexcitation light source 56 are divided into the various wavelength bands(the L band, the C band, and the S band) by the WDM filter 54.

[0127] The light is input into the L band optical amplifier 60, the Cband optical amplifier 61, and the S band optical amplifier 62,respectively, and each amplifies signals in the respective light band.

[0128] The C band optical amplifier 61 is structured from C band opticalamplification units 61-1 and 61-2, splitting couplers 75, adispersion-compensation fiber 53, a variable attenuator 52, automaticgain control circuits 50, and an automatic level control circuit 51. Forthe C band optical amplifier units 61-1 and 61-2, the populationinversion ratio is controlled to about 0.7 by the automatic gain controlcircuits 50. Amplification with substantially constant gain between 1530μm and 1570 μm is achieve.

[0129] The dispersion-compensation fiber 53 is provided to compensatefor dispersion in the transmission path. The variable optical attenuator52 is controlled by the automatic level control circuit 51 to attenuatethe output of the C band optical amplifier unit 61-1 so that the outputof the C band optical amplifier 61 is substantially constant.

[0130] The L band optical amplifier 60 has L band optical amplificationunits 60-1 and 60-2, splitting couplers 75, a dispersion-compensationfiber 53, a variable attenuator 52, automatic gain control circuits 50,and an automatic level control circuit 51.

[0131] For the L band optical amplifier units 60-1 and 60-2, a lowpopulation inversion ratio is maintained by the automatic gain controlcircuits 50. Even though the gain is suppressed to a constant level, thelength of the EDF (which is the amplification medium) is adjusted sothat the gain between 1570 nm and 1610 nm is approximately the same asthat produced in the C band amplifier 61.

[0132] The dispersion-compensation fiber 53 is provided to compensatefor dispersion in the transmission path. The variable optical attenuator52 is controlled by the automatic level control circuit 51 to attenuatethe output of the L band optical amplifier unit 60-1 so that the outputof the L band optical amplifier 60 is substantially constant.

[0133] The S band optical amplifier 62 has S band optical amplificationunits 62-1 and 62-2, splitting couplers 75, a dispersion-compensationfiber 53, a variable attenuator 52, automatic gain control circuits 50,and an automatic level control circuit 51.

[0134] For the S band optical amplifier units 62-1 and 62-2, a highpopulation inversion ratio (around 0.9) is maintained by the automaticgain control circuits 50. Even though the gain is greater than that forthe L-Band amplifier, the length of the EDF (which is the amplificationmedium) is adjusted so that the gain between 1510 nm and 1530 nm isapproximately the same as for the C and L band amplifiers 61 and 60.

[0135] When it comes to the S band optical amplifier units 62-1 and62-2, the specific amplifier structures described previously may beused.

[0136] The dispersion-compensation fiber 53 is provided to compensatefor dispersion in the transmission path. The variable optical attenuator52 is controlled by the automatic level control circuit 51 to attenuatethe output of the S band optical amplifier unit 62-1 so that the outputof the S band optical amplifier 62 is substantially constant.

[0137] The outputs of the L band optical amplifier 60, the C bandoptical amplifier 61, and the S band optical amplifier 62 aremultiplexed with a WDM (wavelength-division multiplexer) coupler 55, andthen output.

[0138]FIG. 13 shows a combination of the S band, the conventional Cband, and the L band, but combinations which employ the other wavelengthbands described with regard to FIG. 1 are also possible.

[0139] The present invention is not limited to the three wavelengthbands described above, but can applied to any combination of two or morewavelength bands. Optical amplifiers that have usable gain in the Sband, the S+ band, and the L+ band can be achieved using silica-basederbium-doped fibers (EDF) by (1) increasing the population inversionratio of the optical amplification medium relative to the L-band opticalamplifiers, (2) expanding the band width over which gain is produced bythe optical amplification medium, (3) equalizing the gaincharacteristics over this band width so as to be able to obtain flatgain characteristics at wavelengths outside the peak gain wavelength ofthe fiber, and (4) selecting the length of the amplification medium toobtain the desired gain value.

[0140] Additionally, the conversion efficiency for converting fromexcited light to signal light can be improved by dividing the opticalamplification medium into multiple segments and placing gain equalizersbetween the segments.

[0141] In addition, similar improvements to the conversion efficiencycan be obtained by if gratings are used for the gain equalizers, withthe gratings formed in the optical wave guide of the opticalamplification medium. This allows for the size of the opticalamplification medium to be reduced.

[0142] One possible benefit of the invention is that it enables theproduction of optical fiber amplifiers for bands other than just the Cband and L band. These new amplifiers could contribute to an increasedtransmission capacity.

[0143] While the invention has been described in connection with thepreferred embodiments and examples, it will be understood thatmodifications within the principles outlined above will be evident tothose skilled in the art. Thus, the invention is not limited to thepreferred embodiments and examples, but is intended to encompass suchmodifications.

What is claimed is:
 1. An optical amplifier comprising: an opticalamplification medium; an excitation source to stimulate theamplification medium to output at least one wavelength gain peak; and again equalizer to equalize the output of the amplification medium suchthat gain is produced at wavelengths other than the wavelength gainpeak.
 2. An optical amplifier according to claim 1, wherein the gainequalizer attenuates gain at the peak wavelength.
 3. An opticalamplifier according to claim 1, wherein the gain equalizer equalizes theoutput of the amplification medium such that nearly even gain isproduced at wavelengths shorter than the wavelength gain peak.
 4. Anoptical amplifier according to claim 1, further comprising: a variableattenuator, and automatic level control circuitry to monitor at leastone of the input of the optical amplifier and the output of the opticalamplifier and maintain the output level of the optical amplifier at asubstantially constant level.
 5. An optical amplifier according to claim1, wherein the optical amplification medium is segmented and comprises aplurality of amplification medium structures which together produce atleast one wavelength gain peak when stimulated by the excitation source.6. An optical amplifier according to claim 5, wherein the amplificationmedium structures are semiconductor optical amplifiers.
 7. An opticalamplifier according to claim 5, wherein the gain equalizer comprises aplurality of gain equalizer segments, which together produce gain atwavelengths other than the wavelength gain peak.
 8. An optical amplifieraccording to claim 7 wherein the gain equalizer segments are positionedwith amplification medium structures positioned therebetween.
 9. Anoptical amplifier according to claim 7 wherein the excitation sourcestimulates the amplification medium with pumping light having a pumpingwavelength, and the gain equalizer segments are substantiallytransparent to the pumping wavelength.
 10. An optical amplifieraccording to claim 1, wherein the optical amplification medium is dopedwith at least one rare earth element.
 11. An optical amplifier accordingto claim 10, wherein the excitation light source stimulates the opticalamplification medium to achieve a population inversion ratio having apositive throughout an optical gain signal wavelength band, thewavelength gain peak is outside of the optical signal wavelength band,and the gain equalizer attenuates the wavelength gain peak.
 12. Anoptical amplifier according to claim 1, wherein the opticalamplification medium has an input and an output, the optical amplifierfurther comprising a feedback loop to the excitation source, to monitorthe input and the output of the amplification medium and maintain asubstantially constant gain within the amplification medium over time.13. An optical amplifier according to claim 1, wherein the opticalamplification medium has an input and an output, the optical amplifierfurther comprising: monitors located at the input and output of theamplification medium to provide feedback; and an automatic gain controlcircuit connected to the monitors to control the excitation source so asto maintain a substantially constant population inversion ratio withinthe amplification medium over time.
 14. An optical amplifier accordingto claim 1, further comprising a resonator, the optical amplificationmedium being located within the resonator.
 15. An optical amplifieraccording to claim 14, wherein the optical amplification medium has aninput and an output, and the resonator comprises: a pair of mirrors thatreflect a selected wavelength; and optical couplers provided at theinput and the output of the amplification medium to divert a portion ofthe light emitted from the optical amplification medium to the mirrors.16. An optical amplifier according to claim 15, wherein the opticalcouplers are 9:1 couplers.
 17. An optical amplifier according to claim15, wherein the mirrors are fiber grating mirrors.
 18. An opticalamplifier according to claim 15, wherein the gain equalizer issubstantially transparent to the selected wavelength.
 19. An opticalamplifier according to claim 15, wherein the selected wavelengthreflected by the mirrors is within a signal band used for opticalsignals to be amplified, and no optical signal is transmitted at theselected wavelength.
 20. An optical amplifier according to claim 1,wherein the excitation source causes excited emission within theamplification medium.
 21. An optical amplifier according to claim 1,wherein the optical amplification medium comprises: a cladding; a dopedcore provided interior to the cladding; and gratings provided within thehighly doped core.
 22. An optical amplifier according to claim 21,wherein the core is highly doped.
 23. An optical amplifier according toclaim 21, wherein the gratings provided within the doped core serve asthe gain equalizer.
 24. An optical amplifier according to claim 21,wherein the gratings are long-period gratings.
 25. An optical amplifierdevice, comprising: an amplification medium comprising at least oneerbium doped fiber; an excitation light source to produce a populationinversion ratio of about 0.7 to about 1.0 within the amplificationmedium; and a gain equalizer to obtain substantially identicalwavelength characteristics for a wavelength band of from about 1490 nmto about 1530 nm.
 26. An optical amplifier device, comprising: anamplification medium comprising at least one erbium doped fiber; anexcitation light source to produce a population inversion ratio of about0.8 to about 1.0 within the amplification medium; and a gain equalizerto obtain substantially identical wavelength characteristics for awavelength band of from about 1450 nm to about 1490 nm.
 27. An opticalamplifier device, comprising: an amplification medium comprising atleast one erbium doped fiber; an excitation light source to produce apopulation inversion ratio of about 0.3 to about 1.0 within theamplification medium; and a gain equalizer to obtain substantiallyidentical wavelength characteristics for a wavelength band of from about1610 nm to about 1650 nm.
 28. An optical amplification method,comprising: selecting a population inversion ratio to achieve positivegain throughout an optical signal wavelength band; exciting theamplification medium to the selected population inversion ration toproduce a wavelength gain peak at a wavelength outside of the opticalsignal wavelength band; equalizing the gain to achieve substantiallyuniform gain over the optical signal wavelength band; and attenuatingamplification in wavelength bands outside of the optical signalwavelength band.
 29. An optical amplification method according to claim28, wherein the optical signal wavelength band is at wavelengths lessthan the wavelength of the wavelength gain peak for the amplificationmedium.
 30. An optical amplifier comprising: a WDM splitter to separatefirst and second different optical signal wavelength bands; an opticalamplification device for the first wavelength optical signal band,comprising: a first amplification medium; an excitation light source toproduce a first population inversion ratio within the firstamplification medium; and a gain equalizer to obtain substantiallyuniform gain over the first optical signal wavelength band; an opticalamplification device for the second wavelength band, comprising: asecond amplification medium; and an excitation light source to produce asecond population inversion ratio within the second amplificationmedium, the first and second population inversion ratios beingdifferent; and a WDM coupler to recombine the first and second opticalwavelength bands after amplification.
 31. An optical amplifier accordingto claim 30, wherein the first population inversion ratio is less thanthe second population inversion ratio.
 32. An optical amplifieraccording to claim 31, wherein the first and second opticalamplification mediums each comprise at least one rare earth elementdoped optical fiber, and the length of the at least one rare earthelement doped optical fiber for the first amplification medium isgreater than that for the second amplification medium.
 33. An opticalamplifier according to claim 30, wherein the first amplification mediumhas a wavelength gain peak, and the wavelength gain peak is outside ofthe first optical signal wavelength band.
 34. An optical amplifieraccording to claim 30, wherein the WDM splitter separates first, secondand third different optical signal wavelength bands, the opticalamplifier further comprising an optical amplification device for thethird wavelength band.